Hello,

in our team we have a problem with changing a tag value in PLC with a button on HMI.
The application controls a process, where several self-made pop-up's requires Operator confirmation.

How does it look in PLC:
In DB there are two bool tags: Confirm_OP1 and Confirm_OP2, both accessible and writable from HMI. If in a step of the process is required the operator confirmation, the Confirm_OP1 tag (or both together) is set to true and the process waits until it will be reset in order to continue.

How does it look in HMI:
The tags are accessed in absolute mode, the acquisition mode is cyclic continuous and the acquisition time is 1s. When the Confirm_OP1’s value changes to true, adequate pop-up with on (or two) buttons are shown. Clicking the button activates few functions and scripts (by the Click event), where two of them are important here. First resets adequate tag Confirm_OP, and second resets tag responsible for the pop-up visibility.

What is the problem:
It have been many times, whether on real equipment or in a simulation, that we had clicked the button, the pop-up’s visibility tag has been reset (the pop-up has disappeared), but the tag in PLC’s DB still has been on true state. It does not matter if it has been click or release event. In an I/O field we have seen that the HMI tag connected with the PLC tag has been reset to 0 and switched by itself back to 1. Shortening the acquisition time to 100ms does not help.

What may be the reason of the problem? Have you any ideas how to solve it so the operation could be confirmed after first operator’s click?

Thank you in advance!

Your question was too long :p

First of all, what's your PLC and HMI model?

Are you modifying the tag value from both end (PLC and HMI) at a same time?

PLC: 1513F-1 PN
HMI: TP1900 Comfort

No, the tag is set on PLC which waits for reset in HMI. I will make sure with my colleague who is responsible for that part if the tag is not sustained somehow.

You need to find the source of modifying the tag to 1. "The PLC tag has been reset to 0 and switched by itself back to 1", that's impossible. Try to find out which one is modifying it, PLC or HMI.

For example, simulate only the PLC and check your logic.

Also a common mistake in the HMI is this: for example, you set a tag in first line and a few lines later you reset it, it will work in HMI but the PLC can't see this changing. If it is your case, set the tag in HMI, use it in PLC then reset it in the PLC.

Guessing the root cause ---

Please check the PLC program. See if the bit that is reset from the HMI is connected to an in-out parameter of a control block (FC / FB).

What do you see?

UPDATE..

Ryan01

You need to find the source of modifying the tag to 1. "The PLC tag has been reset to 0 and switched by itself back to 1", that's impossible. 

qwazee

Guessing the root cause ---

Please check the PLC program. See if the bit that is reset from the HMI is connected to an in-out parameter of a control block (FC / FB).

What do you see?

UPDATE..

Ryan01

You need to find the source of modifying the tag to 1. "The PLC tag has been reset to 0 and switched by itself back to 1", that's impossible. 

Sorry to tell you Ryan that such a scenario is possible in the S7-1200 and S7-1500. The S7-300 and S7-400 does not have this issue. But first I need an answer for my above question.

Do you mean in the S7-1200 and S7-1500, a tag can change its value without getting any instruction?

Ryan01

Do you mean in the S7-1200 and S7-1500, a tag can change its value without getting any instruction?

No.

Communication partners (like HMI) updates data asynchronous to the PLC cycle.

There is a problem when the HMI updates a tag
where that tag is called by value
and being processed inside a code block.

Time analysis

1.  { PLC cycle }
   
   
2. call code block
   
   
3. "myTag" copied to ioTag of code block
   
   
4. code block is being processed
   
   
5. HMI updates "myTag"
   
   
6. code block is finished with processing
   
   
7. ioTag is copied back to "myTag"
   
   
8. code block ends

Result: The update from the HMI was over-written by the executing code block.

Hi qwazee

Now I got your point, I've experienced this before. In other hand, in the S7-300 and S7-400, HMI only sees the last value of a tag (value at the end of the PLC cycle) but in S7-1200 and S7-1500, HMI sees and communicates with PLC even in the middle of PLC cycle.

In this post that was the last thing I'd expect. Anyway, thank you for mentioning this.

Ryan01

Hi qwazee

Now I got your point, I've experienced this before. In other hand, in the S7-300 and S7-400, HMI only sees the last value of a tag (value at the end of the PLC cycle) but in S7-1200 and S7-1500, HMI sees and communicates with PLC even in the middle of PLC cycle.

In this post that was the last thing I'd expect. Anyway, thank you for mentioning this.

Thanks for your reply.

Please go ahead and take the lead on the topic:
What would you recommend to our fellow to solve his problem?
What did your team do when facing the issue?

Hi again,

"Are you modifying the tag value from both end (PLC and HMI) at a same time?"

This is what I asked in my first post.

In this case I try to program in the way to avoid this. But maybe you can recommend another way.